Optical interference mirrors, also referred to as etalons, are well known. In their usual configuration, they consist of a stack of alternating layers of two dielectric materials with differing dielectric constants, i.e., the refractive index differs between the two materials. The dielectric mismatch at the interfaces causes a fraction of the light traversing the stack, usually at a perpendicular angle, to be reflected at each of the successive interfaces. For an interference mirrors, the optical thickness of each of the layers is chosen to be one-quarter of the wavelength of the light to be reflected. The optical thickness equals the product of the physical thickness and the real part of the dielectric constant. Because of the quarter-wavelength thicknesses, the etalon is resonant and the reflected components have coherent phase. A careful study of the mathematics will show that if the resonant conditions are satisfied, no light is transmitted, and, assuming no absorption due to an imaginary part of the dielectric constant, the resonant light is totally reflected.
Of course, the resonant condition is exactly satisfied only for a single wavelength, but in practice the reflective bandwidth is broad enough to reflect a narrow but significant band of wavelength. The remainder of the optical spectrum is incompletely reflected if it is reflected at all.
The reflection at the interface is maximized by a large dielectric mismatch, and absorption is minimized (and hence total reflectivity is maximized) by a purely dielectric material. Therefore, the standard interference mirror consists of a few layers of two dielectrics, such as silica and semi-insulating silicon for the infrared optical band. These materials have real dielectric constants of 3.9 and 11.9 and very small imaginary parts of the dielectric constant.
Nonetheless, recent advances in surface-emitting semiconductor diode lasers have demonstrated the importance of semiconductor interference mirrors. Jewell et al. in U.S. Pat. No. 4,949,350 have disclosed a practical vertical-cavity, surface-emitting semiconductor diode laser including, as illustrated in the cross section of FIG. 1, a pillar 10 epitaxially grown upon a singly crystalline substrate 12. Laterally undefined layers are first grown and later laterally defined into the pillar 10.
The laser pillar 10 includes three major parts: a lower n-type semiconductor interference mirror 14; a resonant cavity region 16; and an upper p-type semiconductor interference mirror 18. All parts are III-V semiconductors of the GaAs/AlAs family, with the active quantum wells being of GaInAs. The resonant cavity region 16 has an optical length equal to the desired free-space wavelength, or to half the wavelength dependent upon the phase matching conditions of the mirrors 14 and 18, or to integral multiples thereof. The resonant cavity region 16 includes on its lower half an n-type region and on its upper half an p-type region, between which is located an active layer 20 having one or more quantum-well layers. Two electrical leads 22 and 24 are attached to the substrate 12 and to a metallization layer 26 so that the active layer 20 can be forward biased so as to emit light. The light is predominantly reflected by the mirrors 14 and 18 and by the metallization 26 so that it lases, and a portion of the lasing light is emitted through the bottom of the substrate 12.
In order to obtain a high-quality active region 20, Jewell et al. grew it epitaxially upon the singly crystalline substrate 12. That is, at least the lower interference mirror 14 is a III-V semiconductor structure epitaxially grown on the substrate 12. However, semiconductor interference mirrors have the disadvantage that the difference in refractive index between the layers is relatively small. Therefore, a large number of stacked layers need to be grown in order to provide the required reflectivity of better than 98%. The surface-emitter diode laser of Jewell et al. had 20 pairs of alternating layers in each of its interference layers and was more than 5 .mu.m high. It requires ten hours to grow by molecular beam epitaxy.
The surface-emitting diode laser of Jewell et al. was based on the GaAs/AlAs material family and hence emitted in the optical band around 0.96 .mu.m. That material family is limited to the 0.77 to 1.0 .mu.m band. On the other hand, the telecommunication industry has a great need for low-power lasers in the 1.3 to 1.55 .mu.m band. The longer wavelength requires a different material family, for example, InP/InGaAsP. However, as discussed by Choa et al. in "High reflectivity 1.55 .mu.m InP/InGaAsP Bragg mirror grown by chemical beam epitaxy," Applied Physics Letters, vol. 59, 1991, pp. 2820-2822, the index difference between the two materials in the interference mirrors is relatively small, and more than forty pairs of alternating layers are required to achieve a reflectivity of 99%. Furthermore, the longer wavelength necessitates thicker quarter-wavelength layers. The resulting single interference mirror has a thickness of 11.5 .mu.m. Such thicknesses are difficult to achieve for epitaxial growth, especially for the quaternary InGaAsP which needs to be lattice matched to InP. Particularly the quaternary layers tend to grow unevenly and the hillocks propagate from layer to layer, producing non-planar mirror layers. Also, a lack of precise compositional control produces quaternary layers that are not precisely lattice matched to the binary layer and substrate. As a result, stress accumulates over the very thick interference mirrors to the point that dislocations occur, thus degrading the electrical and optical performance.
Until now, the longer wavelength surface emitting lasers have not been able to continuously lase at room temperature, presumably because of the poor reflectivity of the mirrors. For example, Tadokoro et al. disclose a diode laser in "Room Temperature Pulsed Operation of 1.5 .mu.m GaInAsP/InP Vertical-Cavity Surface-Emitting Laser," IEEE Photonics Technology Letters, vol. 5, 1992, pp. 409-411. Their laser has mirrors with 34 pairs of InP and GaInAsP, resulting in a reflectivity of 97%. Only pulsed lasing at room temperature is reported and the threshold current is relatively large. However, the mirror of Choa et al. has been used in a pulsed photopumped laser. As reported by Tai et al. in "Room temperature photopumped 1.5 .mu.m quantum well surface emitting lasers with InGaAsP/InP distributed Bragg reflectors," Electronics Letters, vol. 27, 1991, pp. 1540-1542, a 1:4000 duty cycle was required because of heating, and the optical pumping lasing threshold was 100 kW/cm.sup.2.